1. Field of the Invention
This invention relates to oscillators and more particularly pertains to oscillators wherein the frequency of oscillation is determined and controlled by a magnetostatic surface wave delay line.
2. Description of the Prior Art
A common configuration for an oscillator is one which includes an amplifier having an input port, an output port and a feedback path interconnected from the output port to the input port of the amplifier to form a regenerative loop for at least one frequency of oscillation. In general, such a loop will be regenerative and will oscillate at every frequency for which the total phase shift around the loop is either zero or an integral multiple of 2.pi. radians and for which the gain around the loop is at least unity. Amplifiers are generally built to have a predetermined gain and phase shift over a fairly broad range of frequencies. However, for most applications, it is required that the frequency of oscillation of an oscillator remain either reasonably constant at a desired value or that the frequency of oscillation be capable of being varied from one desired value to another in a predictable manner. The common practice is to interconnect a frequency control element in the feedback path around the amplifier when building an oscillator conforming to the description given above. Such a frequency control element must have phase shift and gain characteristics which, when combined with those of the amplifier and all other components interconnected in the loop, cause the loop to be regenerative at a desired frequency of oscillation. An example of such a frequency control element is a delay line. Delay lines are well suited for use as frequency control elements in oscillators for which the desired frequency of oscillation is relatively high.
Surface acoustic wave delay lines have been successfully fabricated to operate at frequencies in the range from about ten megahertz to about one gigahertz although a more practical frequency range for the use of these devices is between fifty and five-hundred megahertz. These delay lines have an advantage in that they can be fabricated to provide a large phase shift in a relatively small, and therefore conveniently useful, package. That is because the speed of propagation of surface acoustic waves on piezoelectric crystals and films is significantly less than the speed of propagation of electromagnetic waves in either free space, waveguides, coaxial cable, or the like.
It is generally accepted that the most practical transducer for converting an electrical signal to a surface acoustic wave propagating on a piezoelectric crystal or film and reconverting the propagated wave to an electrical signal is an interdigital transducer deposited on the surface of the piezoelectric material. As is well-known, the adjacent fingers of the pair of electrodes comprising an interdigital transducer are required to be accurately spaced apart from each other by one-half of a wavelength at a preselected center frequency of operation for the delay line. Also, each such finger is preferably one-quarter of a wavelength wide at this preselected frequency. This fixed geometry for the interdigital input and output transducers is a significant factor in the operation of a surface acoustic wave delay line as a frequency control element when the delay line is interconnected in an oscillator circuit. Well-known photolithographic techniques are used to fabricate interdigital transducers. However, the state-of-the-art of photolithography is limited in terms of its resolution capability. The fabrication of interdigital transducers for use at very small acoustic wavelengths, i.e., at frequencies higher that one gigahertz, is currently regarded as impractical. This is one of the factors which determines the upper frequency limit for the application of surface acoustic wave delay lines. In addition, the propagation loss of surface acoustic wave delay lines increases with increasing frequency. At frequencies above about one gigahertz, the relatively large propagation losses of surface acoustic wave delay lines tends to further reduce their desirability for practical application.
Delay lines in which a signal is propagated as a relatively low velocity wave on the surface of some suitable material have utility in applications other than as frequency control elements for oscillators. The magnetostatic surface wave delay line (MSWDL) has received attention from investigators for signal processing applications in the frequency range from about two gigahertz up to at least eighteen gigahertz. Some practical applications which have been studied or mentioned for MSDWL's are in tunable and tailored filters, pulse compressors, isolators, and delay line equalizers. MSDWL's have the same small-size advantages as surface acoustic wave delay lines due to their relatively low velocity of wave propagation. Although the propagation losses of MSDWL's also increase with increasing signal frequency in the two to eighteen gigahertz frequency range, these propagation losses are less than those of surface acoustic wave devices and this relatively lower propagation loss advantage for the MSDWL increases with increasing signal frequency.